Phantom line of altered pixels in PDF exports

Hi Folks, i’m currently using Indesign CS3. When i export a pdf with an image centered on a white double page spread (say, the right hand page), everything is fine. To check an object position is the same as a previous file, i bring both pdf’s into Photshop auti-locked in the same position and make each layer visible/on and off. Both usually look identical. However, what I’ve noticed that is if move the page down up or down in the ID pages panel (by inserting a preceding page for instance)and then export the pdf… when i bring it into Photoshop, and compare two identical double page spreads… one tends to have two thin horizontal lines of altered pixels. Yet both are identical double page pdf’s exported from the same ID file… just moved up or down the pages panel. It’s a mystery! My export settings are:

High Quality print, Acrobat 5 (pdf 1.4), Compresssion (none/all), Photoshop CS3.

Any insight into why this happens will be gratefully recieved, otherwise i will have to export several version of the finished page spread with the altered lines and visually choose which looks best. (not the end of the world, but good to know why it’s happening).

Thank you.